A 3X9 scope is a fixed power 3X scope with a 9mm objective. A 4X32 scope is a fixed power 4X scope with a 32mm objecive. A 3-9X40 scope is a variable power scope ranging from 3X to 9X with a 40mm objective. There is a correct way to use the terms. Most of the time people can figure out what you mean, but it can cause confusion. They make 8X32 fixed power optics as well as 8-32X variable powered optics as well as several other combinations that would be confusing if written incorrectly.
Why do people sometimes do it? I have no idea, but it is one of those things that causes me minor irritation. Not enough to get upset, but enough to try to educate people of the proper terms.